The present application relates generally to the field of steering devices for vehicles. In particular, the present application relates to rack and pinion steering devices for heavy-duty trucks. Steering systems of vehicles provide a steering force to a steering knuckle, commonly through a portion thereof referred to as the steering arm. The steering knuckle interfaces with the tire and rotates about a kingpin axis to steer the vehicle. A tie rod is coupled to the steering arm in a location that is offset from the kingpin axis, and the tie rod transfers an output steering force from the steering assembly to the steering arm thereby producing a steering torque on the tire.
Axles for use in heavy-duty vehicles (e.g., fire trucks, military trucks, concrete mixer trucks, etc.) are specially designed to accommodate the axle load requirements of these vehicles. Indeed, the gross axle weight rating (GAWR) of an axle often impacts the design of each subcomponent of the axle assembly (e.g., control arms, steering knuckles, steering system, etc.). The weight of the vehicle or the GAWR of the axle impact the design of the steering system where, for example, a heavier vehicle may require a greater output steering force from the steering system. The width of the tires, the area of the contact patch between the tires and a ground surface, and still other factors may also influence the requisite steering force.
The weight of heavy-duty vehicles is much greater than passenger cars or light-duty vehicles. By way of example, a light-duty pickup truck may weigh 6,000 pounds and have a GAWR of 4,000 pounds whereas a fire truck may weigh 72,000 pounds and have a GAWR of 32,000 pounds. Further, heavy-duty vehicles often utilize wider tires than passenger cars or light-duty vehicles thereby producing a larger contact patch and increased resistive friction forces. By way of example only, heavy-duty vehicles may require in excess of 4,500 pounds of steering force to rotate a stationary tire on dry pavement. Traditional steering solutions for light-duty vehicles may not provide the steering forces required in a heavy-duty vehicle. Therefore, specially designed steering assemblies are required in light of, among other factors, the weight and increased contact patch area of heavy-duty vehicles.
Traditional steering systems for heavy-duty vehicles include a steering gear, a pitman arm, and several drag links. The drag links transfer a steering force from the pitman arm to the steering arms to rotate the steering knuckles. While steering gear systems are capable of producing the steering forces a heavy-duty vehicle requires, the axle assembly must be designed to allow for the movement of the various drag links. Facilitating such movement often constrains the design of other components, such as swing arms, side plates, and still other components.
Traditional passenger car and light-duty vehicles utilize rack and pinion steering systems to apply a steering force to the steering arm through tie rods. Rack and pinion steering systems may not require the use of drag link assemblies. Instead, rack and pinion steering systems include an input shaft coupled to both the steering column and a pinion gear. The pinion gear interfaces with a rack to convert rotational movement into lateral movement. An operator's rotation of the steering wheel is transmitted into the pinion gear and produces a lateral output steering force from the rack. In some passenger cars and light-duty vehicle systems, the number of rack gear teeth limits the lateral movement of the rack. Such a configuration prevents operators from continuing to rotate the steering wheel (i.e. over travel the gear rack) because the rack body interferes with the pinion gear teeth. A power steering system may supplement the output steering force produced by an operator with, by way of example, a hydraulic cylinder positioned parallel to the rack. However, systems for passenger cars and light-duty vehicles may not produce the steering forces a heavy-duty vehicle requires. Also, hydraulic cylinders oriented parallel to the rack may be susceptible to damage and debris from the road surface.